Of the various liquid crystal display elements, those using the twisted nematic mode (TM mode) have significant advantages which include low operation voltage, low power consumption and long life, and in recent years have been widely used. A TN-mode cell has anisotropic nematic liquid crystal thin films of a positive dielectric constant interposed between two glass substrates on which transparent electrodes are deposited. The longitudinal axis of the molecules of the liquid crystal are substantially parallel to the planes of the glass substrates. The upper and the lower glass substrates are arranged for a twist angle of almost 90.degree..
FIG. 1 schematically illustrates the arrangement of a conventional TN liquid crystal cell when a voltage is not applied to the cell. FIG. 2 illustrates the arrangement of the liquid crystal cell shown in FIG. 1 when an AC voltage is applied to the cell. In the figures, on the outsides of an upper glass substrate 2a and a lower glass substrate 2b, two polarization plates 1a and 1b are arranged, respectively, so that their transmission or absorption axes align with the direction of arrangement of molecules of the liquid crystal on the respective side of the cell. Transparent electrodes 3a and 3b, formed of a transparent conductive film such as, for example, an indium tin oxide (ITO) film, doped with tin oxide, are deposited, respectively, on the insides of the upper glass substrate 2a and the lower glass substrate 2b. After the electrodes, orientation films 4a and 4b are applied. A nematic liquid crystal layer 5 wherein the molecules 6 are represented as cylinders is interposed between the two glass substrates on which the transparent electrodes 3a and 3b are deposited and the orientation films 4a and 4b are applied.
FIG. 1 illustrates a state wherein the orientation film 4a on the glass substrate 2a is rubbed in one direction to align the longitudinal axes of the molecules 6 of the liquid crystal and to tilt the molecules 6 of the liquid crystal at an angle, that is, pre-tilt angle .alpha..sub.0, illustrated by a small angle of the molecules with respect to the plane of the substrate when the voltage is not applied. If an AC voltage from a source 7 is applied to the liquid crystal panel shown in FIG. 1, each molecule 6 of the liquid crystal in the middle of the cross section of the cell, as shown in FIG. 2, rises in one direction so that the angle with respect to the plane of the glass substrates is increased.
If the polarization axes of the polarizing plates 1a and 1b are parallel to each other, when the voltage is not applied, as in FIG. 1, incident light becomes linearly polarized light at the polarizing plate 1b, changes the direction of its polarization by an angle of 90.degree. due to the twist of the molecules of the liquid crystal layer 5, and a dark state exists because the incident light is parallel to an absorption axis of the polarizing plate 1a, through which the light must exit. On the other hand, when the voltage is applied as in FIG. 2, the incident light which has been changed to linearly polarized light at the polarizing plate 1b reaches the polarizing plate 1a on the outgoing side with the same direction of polarization and a bright state exists because most of the incident light passes through the polarizing plate 1a.
If the polarization axes of the polarizing plates 1a and 1b are arranged so that they are orthogonal to each other, the dark and bright state for the liquid crystal cell are reversed with respect to the application of a voltage to the cell.
If AC voltage from source 7, shown in FIG. 2, is set to, for example, 5 V and the glass substrates 2a and 2b are observed from the upper and lower angles of view to measure a contrast ratio between dark and light states of the liquid crystal cell, then the graphs shown in FIG. 3 and FIG. 4 are obtained. In FIG. 3, U30 or D30 represent an angle of 30.degree. from the upper or the lower side, respectively. In FIG. 4, the solid line and the dashed line represent the results of measurement from the upper and lower sides, respectively.
A problem associated with the conventional TN-LCD described above, is that the best angle of view cannot be set in the direction normal to the plane of the glass substrates. If the direction of the best angle of view from the glass substrate is defined as upward, the angle of view is symmetric in the horizontal direction. However, the angles of view, as shown in FIG. 3 and FIG. 4, are unsymmetrical in the vertical direction. To solve the problem, a method for covering the panel with a Fresnel lens was contrived. However, the method has its faults in that the presence of stripes is disagreeable and the costs are rising. Further, another method for forming micro irregularities on the surface of one side of the panel for enlarging the angle of view has been contrived. However, the method also has its faults in that roughness in the perception by the eyes is unavoidable and costs are increased by the necessity of forming the micro irregularities.